Vestibulo-ocular reflexes (VORs) are responsible for maintaining stability of images on the retina during head movements. Many of the behavioral features and neurophysiological mechanisms underlying control of angular VORs have been elucidated. Comparatively little is known about the organization of otolith-ocular reflexes Or their adaptive capabilities. Otolith-ocular reflexes generate three types of compensatory eye movements: 1) ocular counterrolling in response to dynamic and static head tilt, 2) linear VORs in response to dynamic linear movements, and 3) nystagmus induced by constant velocity off vertical axis rotation. Vestibular nerve afferents innervating the otoliths cannot distinguish between movements leading to each of these three responses. Research described in this project will define mechanisms involved in the signal processing leading to these three responses. The capabilities and mechanisms involved in the adaptation of the otolith-ocular reflexes will also be studied as an example of motor learning. The long-term goal of this research is-to define the mechanisms involved in the specification and adaptive control of otolith-ocular reflexes, with the practical aim of improving the diagnosis and treatment (rehabilitation) of patients with vestibular disorders. The specific objectives of this proposal are l) to define the physiological properties of central vestibular neurons that receive both semicircular canal and otolith afferents in squirrel monkeys and 2) to define the mechanisms responsible for adaptive control of angular and linear VORs in both squirrel monkeys and humans. Three dimensional and vergence eye movements in both monkeys and humans, as well as single-unit activity in the vestibular nuclei of the squirrel monkeys, will be recorded. Otolith-ocular reflexes will be elicited in response to static and dynamic head tilt, eccentric rotations, and linear translations. The role of angular velocity signals, phasic-tonic responses of irregular otolith afferents, and frequency-specific filtering of otolith signals in these processes will be determined in monkeys through bilateral semicircular canal plugging and functional ablation of the irregular afferents. Adaptation will be elicited using visual-vestibular conflict paradigms. Transfer of adapted VOR gain to reflexes induced by differing combinations of semicircular canal and otolith activity will identify points of convergence in these pathways and define mechanisms involved in the generation of angular velocity signals from otolith activity.